1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and especially to a method of forming resist holes using lithography techniques.
2. Description of the Background Art
As semiconductor device dimensions have been more and more decreasing in recent years, in terms of control of resist hole dimensions by lithography techniques, it is getting increasingly difficult to form resist holes of the same diameter (the resist hole diameter is hereinafter referred to as a xe2x80x9chole diameterxe2x80x9d). The resist hole is a hole formed in a resist, the resist being a masking material in formation of a contact hole in an interlayer film on a semiconductor substrate, for example. It is hereinafter simply referred to as a xe2x80x9cholexe2x80x9d.
In formation of holes in a resist by a projection aligner, the hole diameter may have pitch dependence depending on lighting conditions of the projection aligner. The pitch dependence represents a characteristic that when a resist has a plurality of holes, the hole diameter varies depending on the center-to-center distance of adjacent holes (hereinafter referred to as a xe2x80x9chole pitchxe2x80x9d).
Now, the pitch dependence will be set forth in more detail. FIGS. 49 and 50 are graphs of the relationship between the hole pitch and the hole diameter in conventional techniques, the graphs illustrating simulation results in consideration of only an optical system in a projection aligner. A laser used in the simulations is a KrF excimer laser operating at wavelengths of 248 nm, and the dimensions of holes in a masking material for use in formation of holes in a resist are 0.22 xcexcm. A desired design value for the hole diameter is 0.16 xcexcm.
FIG. 49 is a graph for a constant NA (NA=0.6) and "sgr"=0.6, 0.7, and 0.8; and FIG. 50 is a graph for a constant "sgr" ("sgr"=0.6) and NA=0.6, 0.65, and 0.7. NA is the numerical aperture of a lens between a resist formed on the semiconductor substrate, and a masking material for use in formation of holes in a resist. NA represents the resolution of the lens. "sgr" is a constant given by
"sgr"=iNA/NAxe2x80x83xe2x80x83(1)
where iNA is the numerical aperture for light incident on the masking material.
As shown in FIG. 49, under the lighting conditions of NA=0.6 and "sgr"=0.6, the hole diameter, when the hole pitch is between 0.4 and 0.5 xcexcm, is mostly smaller than the desired design value of 0.16 xcexcm, and as the hole pitch increases, the hole diameter mostly increases. For the hole pitch between 0.5 and 1.0 xcexcm, the hole diameter is nearly equal to the desired design value of 0.16 xcexcm. Under the lighting conditions of NA=0.6 and "sgr"=0.7, the hole diameter, when the hole pitch is between 0.4 and 0.5 xcexcm, is slightly larger than the desired design value of 0.16 xcexcm, but as a whole, it is nearly equal to the desired design value of 0.16 xcexcm. Under the lighting conditions of NA=0.6 and "sgr"=0.8, the hole diameter, when the hole pitch is between 0.4 and 0.6 xcexcm, is larger than the desired design value of 0.16 xcexcm, and as the hole pitch increases, the hole diameter mostly decreases. For the hole pitch between 0.6 and 1.0 xcexcm, the hole diameter is nearly equal to the desired design value of 0.16 xcexcm.
On the other hand, as shown in FIG. 50, under the lighting conditions of NA=0.6 and "sgr"=0.6, the hole diameter, when the hole pitch is between 0.4 and 0.5 xcexcm, is mostly smaller than the desired design value of 0.16 xcexcm and as the hole pitch increases, the hole diameter mostly increases. For the hole pitch between 0.5 and 1.0 xcexcm, the hole diameter is nearly equal to the desired design value of 0.16 xcexcm. Under the lighting conditions of NA=0.65 and "sgr"=0.6, the hole diameter, when the hole pitch between 0.4 and 0.5 xcexcm, is slightly smaller than the desired design value of 0.16 xcexcm, but as a whole, it is nearly equal to the desired design value of 0.16 xcexcm. Under the lighting conditions of NA=0.7 and "sgr"=0.6, the hole diameter is nearly equal to the desired design value, 0.16 xcexcm.
As above described, under some lighting conditions (the values NA and "sgr") of the projection aligner and for the hole pitch between 0.4 and 0.5 xcexcm or between 0.4 and 0.6 xcexcm, the hole diameter has a characteristic of depending on the hole pitch. The aforementioned xe2x80x9cpitch dependencexe2x80x9d refers to this characteristic.
As shown in FIGS. 49 and 50, the pitch dependence of the hole diameter can be virtually eliminated by the settings of NA=0.6 and "sgr"=0.7 or the settings of NA=0.7 and "sgr"=0.6. The values NA and "sgr", however, exercise influence on the resolution and focal depth of a lens for focusing light on the resist and therefore, they cannot be determined for only the purpose of eliminating the pitch dependence. Besides, the values NA and "sgr" may sometimes be fixed by the specifications of the projection aligner, in which case they cannot be determined freely on the manufacturing end of the semiconductor device. That is, the hole diameters of holes formed in a resist may have pitch dependence depending on the required resolution and focal depth and depending on the projection aligner to be used.
Let the lighting conditions be NA=0.6 and "sgr"=0.8 based on the required resolution and focal depth and on a projection aligner to be used. At this time, as shown in FIG. 49, the hole diameter, when the hole pitch is between 0.4 and 0.6 xcexcm, mostly decreases with increasing hole pitch. Use of a projection aligner with a small "sgr" for reduction of such pitch dependence creates a problem of reducing the resolution of a lens. Use of a projection aligner with a large NA for reduction of the pitch dependence creates a problem of reducing the focal depth of a lens; in addition, such a projection aligner with a large NA is expensive and therefore, reducing the pitch dependence through the use of such a projection aligner becomes a cause of increasing the semiconductor device manufacturing cost. While in the above description, the lighting conditions of NA=0.6 and "sgr"=0.8 are presented as conditions for developing the pitch dependence which represents the characteristic that the hole diameter decreases with increasing hole pitch, the hole diameter also varies according to the type of a resist to be used and to the diameters of holes in a masking material for use in formation of holes in a resist. Therefore, not only the lighting conditions of NA=0.6 and "sgr"=0.8 but also various other conditions develop the above pitch dependence.
A first aspect of the present invention is directed to a method of manufacturing a semiconductor device, comprising the steps of: (a) forming a resist on an object to be etched; (b) forming in the resist a plurality of holes a pitch of which includes different pitches; and (c) performing ion implantation on the resist to reduce pitch dependence caused to diameters of the holes.
According to a second aspect of the present invention, the method of manufacturing a semiconductor device of the first aspect further comprises the step of: (d) between the steps (b) and (c), performing processing for reduction of the diameters of the holes.
A third aspect of the present invention is directed to a method of manufacturing a semiconductor device, comprising the steps of: (a) forming a resist on an object to be etched; and (b) forming a plurality of holes in the resist, wherein the holes include a first hole of a first pitch and a second hole of a second pitch larger than the first pitch. The method further comprises the step of: (c) performing ion implantation on the resist to make the amount of increase in the diameter of the second hole larger than that in the diameter of the first hole.
A fourth aspect of the present invention is directed to a method of manufacturing a semiconductor device, comprising the steps of: (a) forming a resist on an object to be etched; (b) forming in the resist a plurality of holes having center-to-center distance of not more than 0.5 xcexcm between holes and having diameters smaller than a desired diameter, and (c) performing ion implantation on the resist to increase the diameters of the holes to the desired diameter.
According to a fifth aspect of the present invention, in the method of manufacturing a semiconductor device of the first through fourth aspects, an ion used for the ion implantation is an ion of an inert gas.
In the fifth aspect of the present invention, the ion is preferably an Ar ion.
In the method of manufacturing a semiconductor device according to the first aspect of the present invention, the diameters of holes of great pitches increase by a larger amount than the diameters of holes of small pitches. This allows a reduction in pitch dependence caused to the hole diameters.
In the method of manufacturing a semiconductor device according to the second aspect of the present invention, the reduction processing in the step (d) and the ion implantation in the step (c) can improve resistance of the resist to etching and can reduce a difference between the hole diameter in the step (b) and the hole diameter after the ion implantation in the step (c).
In the method of manufacturing a semiconductor device according to the third aspect of the present invention, ion implantation in the resist having holes increases the hole diameter according to the pitch. Consequently, it is possible to increase the hole diameter while maintaining insulation between holes of small pitches.
In formation of holes having center-to-center distances of 0.5 xcexcm or less in the resist, various optical problems arise if the hole diameters are increased to a desired value by varying lighting conditions of a projection aligner. In the method of manufacturing a semiconductor device according to the fourth aspect of the present invention, however, the hole diameters are increased to a desired value by ion implantation in the resist. This minimizes the occurrence of such optical problems.
In the method of manufacturing a semiconductor device according to the fifth aspect of the present invention, use of ions other than an inert gas for ion implantation degrades semiconductor device characteristics because of ion penetration in the resist or ion diffusion in the object to be etched. In the present invention, however, since an ion of an inert gas is used for ion implantation, there occur less ion penetration in the resist and less ion diffusion in the object to be etched. Accordingly, there is less degradation in semiconductor device characteristics than when using an ion of a gas other than inert gases.
An object of the present invention is to provide a method of manufacturing a semiconductor device that reduces pitch dependence which represents a characteristic that the hole diameter decreases with increasing hole pitch.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.